1. Technical Field
The present invention relates to an apparatus for helping to protect a vehicle occupant and, in particular, to a gas generator usable in a seat belt webbing pretensioner.
2. Description of the Prior Art
A known vehicle seat belt system includes a retractor on which seat belt webbing is wound. The retractor has a spool, which is rotatable in a belt withdrawal direction and in an opposite belt retraction direction. The seat belt system also includes a pretensioner. The pretensioner includes a gas generator, which is electrically actuatable to produce fluid under pressure to move a piston in a cylinder. Rack teeth formed on the piston are in meshing engagement with a pinion. Upon actuation of the gas generator, fluid under pressure causes linear movement of the piston, and the rack teeth move linearly to rotate the pinion in a first direction. Rotation of the pinion in the first direction results in rotation of the seat belt webbing spool in a first direction, to remove slack from the seat belt webbing.
The gas generator includes a body of solid propellant and an electrically actuatable initiator for igniting the propellant to generate gas under pressure. Upon actuation of the gas generator, the pressurized gas causes the piston to move to the opposite end of the cylinder, resulting in rotation of the spool in the first direction.
One known gas generator for a seat belt pretensioner includes a molded plastic shell to enclose the propellant. Because the shell is plastic, it electrically isolates the propellant, but it does not hermetically seal the propellant. In addition, the plastic shell breaks into fragments upon ignition of the propellant, and the fragments must be filtered.
Another known gas generator for a seat belt pretensioner has a metal shell, which encloses and hermetically seals the propellant and an electrically energizable bridgewire. The metal shell ruptures upon ignition of the propellant and thus does not produce fragments that must be filtered. The metal shell, however, does not electrically isolate the bridgewire and propellant.